Wavelength monitored and stabilized source

ABSTRACT

Methods and apparatus for sampling techniques can constantly monitor a spectral output from a broadband source in order to control a central wavelength of interrogation light supplied by the source for input to a sensor. A first portion of light output from the broadband source passes through a controller module for spectral analysis and referencing to provide measurements that can be used as feedback to actively modify a second portion of the light from the source. This modified second portion thereby controls the central wavelength to ensure accurate determination of sensor response signals received at a receiver.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.11/855,297, filed Sep. 14, 2007, which is herein incorporated byreference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

Embodiments of the invention generally relate to optical sensor systemsand, more particularly, to improving wavelength stability in broadbandsource light used to interrogate optical sensors.

2. Description of the Related Art

Optical sensor systems operate by exposing a portion of an opticalwaveguide to an environmental condition that modulates a light signaltransmitted within the optical waveguide. This modulation alters one ormore parameters of the light transmitted within the optical waveguide,such as amplitude, power distribution versus frequency/wavelength,phase, or polarization. Analyzing modulated light emerging from thewaveguide enables determining values indicative of the environmentalcondition. Such systems utilize sensors based on, for example, Bragggratings or interferometers, to measure a wide variety of parameters,such as strain, displacement, velocity, acceleration, flow, corrosion,chemical composition, temperature or pressure. In one example of anoptical sensor system, a fiber optic gyroscope (FOG) enables measuringangular rotation since application of force alters the wavelength oflight as it travels through a sensing coil of an optical fiber, therebyproducing phase changes from which measurements can be made.

Instabilities in a center wavelength of input light provided by abroadband light source may cause variations in sensor response signalsproduced upon the interrogating light arriving at the optical sensor.For example, broadband sources producing input light without a stablecenter wavelength when used with a Bragg grating sensor may causevariations in the reflected response signal emitted by the sensor,resulting in incorrect measurements or undesirable noise. In the FOG,the phase change with acceleration depends on wavelength such that anychange in the center wavelength of the broadband source input into aninterferometer of the FOG produces drifts in a scalar factor associatedwith the acceleration and wavelength. Accurate and reliable measurementsdetermined by detection of response signals from the optical sensorsrequire a broadband light source outputting light with a centerwavelength that does not drift around with time or other environmentalchanges. However, attempts in many environments to achieve such a stablebroadband light source by stabilization and control (e.g., temperaturestabilization or vibration dampening) of components proves difficult,expensive and oftentimes insufficient.

Therefore, there exists a need for optical sensing configurations andmethods that improve wavelength stability of input broadband light usedto interrogate an optical sensor which may include an FOG device.

SUMMARY OF THE INVENTION

In one embodiment, an optical system for producing a stabilizedbroadband light output to a sensor includes a broadband light source forproducing broadband light signals and a splitter dividing the lightsignals into first and second portions along first and second outputpathways, respectively. A controller module having a sweeping tunablefilter coupled to the first output pathway of the splitter receives thefirst portion of the light signals prior to outputting respectivefiltered light portions to a comb filter and a wavelength referenceelement, wherein control circuitry is configured to evaluate detectedsignals from the comb filter and the reference element to generate acontrol signal output. A wavelength dependent variable attenuatorcoupled to the second output pathway of the splitter and the controlcircuitry receives the second portion of the light signals and thecontrol signal output, wherein the attenuator is configured to modifythe second portion of the light signals based on the control signaloutput, thereby providing the stabilized broadband light output.

For one embodiment, an optical system includes a broadband light sourcefor producing broadband light signals and a splitter dividing the lightsignals into first and second portions. A sweeping tunable filtercoupled to the splitter receives the first portion of the light signalsprior to outputting respective filtered light portions to a comb filterand a wavelength reference element. Control circuitry configured toevaluate detected signals from the comb filter and the reference elementgenerates a control signal output. A spectrum modifier coupled to thesplitter and the control circuitry receives the second portion of thelight signals and the control signal output, wherein the modifier isconfigured to adjust the second portion of the light signals based onthe control signal output, thereby providing a stabilized broadbandlight output. A sensor element couples to the modifier and is configuredto provide response signals from interrogation by the stabilizedbroadband light output that is unswept in time across wavelengthsproduced by the source. A receiver couples to the sensor element and isconfigured to detect and process the response signals.

According to one embodiment, a method of stabilizing broadband lightoutput to a sensor includes generating a broadband light and dividingthe light into first and second pathways, wherein a controller modulewavelength scans light propagating in the first pathway prior tooutputting respective filtered light portions to a comb filter and awavelength reference element of the controller module. The methodfurther includes generating a control signal output with controlcircuitry based on detected signals from the comb filter and thereference element. Modifying light propagating in the second pathwaybased on the control signal output produces the stabilized broadbandlight output.

BRIEF DESCRIPTION OF THE DRAWINGS

So that the manner in which the above recited features of the presentinvention can be understood in detail, a more particular description ofthe invention, briefly summarized above, may be had by reference toembodiments, some of which are illustrated in the appended drawings. Itis to be noted, however, that the appended drawings illustrate onlytypical embodiments of this invention and are therefore not to beconsidered limiting of its scope, for the invention may admit to otherequally effective embodiments.

FIG. 1 is a schematic process map of an optical system for producing astabilized broadband light output in accordance with embodiments of theinvention.

FIG. 2 is a block diagram of an exemplary optical sensor system inaccordance with embodiments of the invention.

FIG. 3 is a flow process for stabilizing broadband light output to asensor in accordance with embodiments of the invention.

DETAILED DESCRIPTION

Embodiments of the invention relate to sampling techniques which canconstantly monitor a spectral output from a broadband source in order tocontrol a central wavelength of interrogation light supplied by thesource for input to a sensor. A first portion of light output from thebroadband source passes through a controller module for spectralanalysis and referencing to provide measurements that can be used asfeedback to actively modify a second portion of the light from thesource. This modified second portion thereby controls the centralwavelength to ensure accurate determination of sensor response signalsreceived at a receiver. In some embodiments, the sensor response signalsmay be from a fiber-optic gyroscope benefiting from the centerwavelength being stabilized, as discussed herein.

FIG. 1 shows a schematic process map of an optical system 100 forproducing a stabilized broadband light output 116. The system 100includes a broadband light source 102, a tunable filter 104, first,second and third detector circuits 106, 108, 110, a data processor 112and a wavelength dependent variable attenuator 114, which is controlledby signals generated with the data processor 112 to produce the output116 derived from light signals provided by the source 102. The source102, e.g., an amplified spontaneous emission (ASE) source, produces thelight signals unswept in time across wavelengths and defining abroadband optical spectrum including wavelengths which may range, forexample, at least 10 nanometer (nm) or at least 50 nm. The shape of thespectrum may change or drift over time. This instability causes changesin the center wavelength, which is critical in determining output from asensor (see, FIG. 2). Therefore, control of the attenuator 114 by theprocessor 112 ensures that the center wavelength of light from thesource 102 is maintained by wavelength and amplitude stabilizing thelight prior to being output for use in interrogating the sensor.

In operation, a first portion of the light from the source 102 bypassesthe tunable filter 104 and enters the attenuator 114. A second portionof light from the source 102 passes through the tunable filter 104 priorto splitting into the first detector circuit 106, the second detectorcircuit 108, and the third detector circuit 110 that is optional. Thetunable filter 104 may sweep across all wavelengths of the spectrum ofthe source 102 for whole spectrum measurement and analysis. Examples ofsuitable tunable filters include a piezoelectrically tunable Fabry-Perot(F-P) filter, a tunable acousto-optic filter, or a tunable interferencefilter. The processor 112 may control the tunable filter 104 tofacilitate synchronization of measurements taken with the processor 112based on signals from the detector circuits 106, 108, 110.

The first detector circuit 106 includes a comb filter, such as an F-Petalon with fixed and known free spectral range, which produces areference comb spectrum with peaks having a constant, known frequencyseparation equal to the free spectral range to provide an accuratefrequency/wavelength scale. The second detector circuit 108 provides anaccurate wavelength reference by, for example, passing light onto atleast one fiber Bragg grating (FBG) with a known wavelength. Someembodiments can utilize a reference interference filter without areference FBG by, for example, using a source envelope to identify oneor more reference peaks in the comb spectrum itself for absolutewavelength referencing.

The signals detected in the first and second detector circuits 106, 108are simultaneously sampled, processed and compared in the data processor112, providing accurate and repeatable wavelength measurement across thewhole measured spectrum. At the same time, the third detector circuit110 measures spectral power of the light as received in the thirddetector circuit 110 for correlation to the wavelength measurement. Thisspectral power may therefore be derived from the first and seconddetector circuits 106, 108 if power measurements are performed inaddition to detecting the comb spectrum and Bragg wavelength. Thedetected signals in combination from the detector circuits 106, 108, 110therefore enable monitoring and measuring the optical spectrum of thesource 102. If any changes in the spectrum are measured, signalsgenerated by the data processor 112 can control the attenuator 114 toalter attenuation selectively for certain wavelengths of the firstportion of light received at the attenuator 114 from the source 102. Thecontrol of the attenuator 114 can maintain an identified centerwavelength for the output 116.

FIG. 2 illustrates a block diagram of an exemplary optical sensor system200. The system 200 exemplifies an architecture employing concepts ofthe process map shown in FIG. 1. Components of the system 200 include abroadband light source 202, a controller module 219, a spectrum modifier214, a sensor and a sensor response detector and processor 220. For someembodiments, the controller module 219 includes a tunable filter 204, anF-P etalon 211, a Bragg grating reference 215, a comb detector 206, astable reference artifact detector 208, and a data processor 212.

Light from the source 202 travels to an initial tap or splitter 203 thatsplits the light into two paths. For some embodiments, the entirespectrum of the light from the source 202 passes continuously throughthe initial splitter 203 to the controller module 219 along controlleroptical fiber 205 and to a lead optical fiber 207 coupled to themodifier 214. The tunable filter of the controller module 219 mayprovide the only wavelength scanning in the system 200 such that sensorinterrogating light that does not pass through the controller module 219may bypass any wavelength scanning of the light from the source 202. Thecontroller optical fiber 205 couples to the tunable filter 204 thatwavelength scans the light to provide filtered light. A detectioncircuit splitter 209 couples the F-P etalon 211 and the Bragg gratingreference 215 to the tunable filter 204 and divides the filtered lightfrom the tunable filter 204 to each. A coupler or circulator 213 couplesthe Bragg grating reference 215 to the artifact detector 208. The comband artifact detectors 206, 208 respectively sense outputs from the F-Petalon 211 and the Bragg grating reference 215. The data processor 212receives detected signals from the comb and artifact detectors 206, 208and evaluates a spectrum of the source 202 based on the detected signalsas described heretofore.

The data processor generates control signals 217 input as operatinginstructions into the spectrum modifier 214 to regulate functioning ofthe modifier 214. The control signals 217 may instruct the modifier 214to adjust variable attenuation or amplification of certain wavelengthsor dropping of certain wavelengths to ensure that the spectrum of thesource 202 as received by the modifier 214 via lead optical fiber 207 isadjusted in a manner that produces a stabilized broadband light outputthrough a sensing string 216 to the sensor 218. For some embodiments,the stabilization may include wavelength and amplitude stabilization andmay maintain an identified mean center wavelength. This stabilizedbroadband light output transmitted through the sensing string 216interrogates the sensor 218 and may contain at one time substantiallyall wavelengths produced by the source 202.

For example, the control signals 217 may instruct the modifier 214 topass the light from the source 202 without alteration if the spectrumevaluated by the processor 212 already has the identified centerwavelength. However, the control signals 217 may instruct the modifier214 to attenuate wavelengths, such as 1530 nm to 1535 nm 10%, to obtainthe identified center wavelength when the spectrum evaluated by theprocessor 212 has a shifted center wavelength different from theidentified center wavelength. This example illustrates the ability tocontrol broadband interrogation light with accuracy and in real time.

The sensor string 216 couples to the sensor 218 shown as an opticalfiber sensing coil containing between 200 meters and 5.0 kilometers offiber to form an interferometric fiber-optic gyroscope (IFOG). Inoperation, the stabilized broadband light output launches into thesensor 218. Rotation of the sensor 218 affects the light, therebygenerating response light signals. The response light signals from thesensor 218 propagate to the sensor response detector and processor 220that then receives the response light signals for measuring rotation ofthe sensor 218. Determinations of the rotation or other parameterobtained utilizing techniques as described herein may be transmitted asan output 222 to a user via, for example, a display or printout.Further, the output 222 may be used to generate a signal or control adevice.

FIG. 3 depicts a flow process 300 for stabilizing broadband light outputto a sensor utilizing systems such as described herein. The process 300begins at a light generating step 302 where light is emitted from abroadband source. At monitoring tap step 304, dividing the light intofirst and second pathways occurs with light propagating in the firstpathway being wavelength scanned to provide filtered light. Themonitoring tap step 304 further includes outputting respective portionsof the filtered light to a comb filter and a wavelength referenceelement. Instruction step 306 generates a control signal output based ondetected signals from the comb filter and the reference element usingcontrol circuitry. The detected signals provide an indication of aspectrum of the light emitted by the source. Modifying light propagatingin the second pathway occurs at spectrum stabilization step 308 based onthe control signal output. Modified light produced at step 308 providesa stabilized broadband sensor interrogation light for interrogating asensor.

While the foregoing is directed to embodiments of the present invention,other and further embodiments of the invention may be devised withoutdeparting from the basic scope thereof, and the scope thereof isdetermined by the claims that follow.

1. An optical system, comprising: a broadband light source for producingbroadband light signals; a splitter dividing the light signals intofirst and second portions; a sweeping tunable filter coupled to thesplitter to receive the first portion of the light signals prior tooutputting respective filtered light portions to a comb filter and awavelength reference element; control circuitry configured to evaluatedetected signals from the comb filter and the reference element togenerate a control signal output; a spectrum modifier coupled to thesplitter and the control circuitry to receive the second portion of thelight signals and the control signal output, wherein the modifier isconfigured to adjust the second portion of the light signals based onthe control signal output, thereby providing a stabilized broadbandlight output; and a sensor element coupled to the modifier andconfigured to provide response signals from interrogation by thestabilized broadband light output that is unswept in time acrosswavelengths produced by the source; a receiver coupled to the sensorelement and configured to detect and process the response signals. 2.The system of claim 1, wherein the sensor element includes a fiber opticgyroscope.
 3. The system of claim 1, the modifier is configured toadjust the second portion of the light signals to maintain a meancentral wavelength of the stabilized broadband light output.
 4. Thesystem of claim 1, the modifier is configured to adjust the secondportion of the light signals to maintain an identified spectral shape ofthe stabilized broadband light output.
 5. The system of claim 1, whereinthe wavelength reference element includes a Bragg grating and the combfilter includes a Fabry-Perot etalon.
 6. The system of claim 1, whereinthe source is an amplified spontaneous emission source.
 7. The system ofclaim 1, wherein the stabilized broadband light output includessubstantially all wavelengths produced by the source at one time.